1. Field of the Invention
The present invention relates to a color image forming apparatus for forming a color image on a sheet by using toners, and a color image forming method there of, and more particularly to a color image forming apparatus which has a tandem type engine arranged a plurality of toner image forming units, and a color image forming method thereof.
2. Description of the Related Art
A tandem type color image forming apparatus as a color image forming apparatus, which has a plurality of toner image forming units parallel-disposed in a carrying path thereof, continuously forms the toner images with different colors on a sheet to enable a high-speed printing. FIG. 7 shows a conventional tandem type color electro-photographic apparatus.
In FIG. 7, reference numbers 10, 20, 30 and 40 indicate the OPC (Organic Photoconductor) drums of the Yellow-color, Magenda-color, Cyan-color, and Black-color toner process units, respectively. The electrostatic latent images are formed on the OPC drums 10,20,30 and 40 and developed with Yellow-color, Magenda-color, Cyan-color, and Black-color toners by the unshown developing members in the Yellow-color, Magenda-color, Cyan-color, and Black-color toner process units.
The developed toners are transferred onto a sheet 100 by the strength of an electric field, which has been generated between the OPC drums 10 to 40 and the sheet 100 by a voltage applied from transfer members 80 to 84 such as transfer rollers, etc. The sheet 100 is electrically charged by a sheet adsorption roller 60, and then it is adsorbed onto a dielectric belt 50.
The sheet 100 is carried to the transfer positions of the OPC drums 10, 20, 30, and 40 by the movement of the dielectric belt 50, and all of the four colors are transferred onto the sheet 100. Then, the sheet 100 is taken off the dielectric belt 50, and the toner images on the sheet 100 are fixed by an unshown fixing member. Even when the four colors are transferred at different positions, the dielectric belt 50 adsorbs the charged sheet 100, so that a high-quality color image can be formed without a position deviation of each color on the sheet 100.
As disclosed in U.S. Pat. No. 5,907,758 (Japanese Unexamined Published Patent 10-198120), and U.S. Pat. No. 6,021,286 (Japanese Unexamined Published Patent 11-161035), etc., in a conventional tandem type color electro-photographic process, the dielectric belt 50 is charged to a high electric potential such as approximately 1000V by a charging device 70. The reason why the dielectric belt 50 is charged to the high electric potential is explained. An electric potential difference between the sheet 100 and the OPC drums 10 to 40 can be increased for the charged electricity of the dielectric belt 50, even when the transfer voltage applied to the four color toner transfer members 80, 81, 82, and 83 is lowered. The strength of the electric field generated between the sheet 100 and the OPC drum is caused by increasing the potential difference between the sheet 100 and the OPC drum to a degree that no electric discharge occurs, so that the transfer efficiency can be improved.
The above process is explained below, taking an example. The conductive brush 70 charges the dielectric belt 50 to 1000V. At the same time, the sheet adsorption roller 60 charges the sheet 100 to adsorb the sheet 100 onto the dielectric belt 50. At this time, the sheet 100 must be charged so that the potential difference between the front and back faces of the sheet 100 can be set to 2000V.
Then, the Yellow-color toners contained in the OPC drum 10 are transferred onto the sheet 100 in the Yellow-color toner transfer position. A voltage of xe2x88x92100V (direct current) is applied to a transfer roller 80. At this time, electric charges on the sheet move to a photosensitive body 10. Therefore, the potential difference between the front and back faces of the dielectric belt 50 is lowered from 1000V to 400V.
Then, the Magenda-color toners contained in the OPC drum 20 are transferred onto the sheet 100 in the Magenda-color toner transfer position. A voltage of 500V (direct current) is applied to a transfer roller 82. At this time, the electric charges move to a photosensitive body 20. Therefore, the potential difference between the front and back faces of the dielectric belt 50 is lowered from 400V to 200V.
Then, the Cyan-color toners contained in the OPC drum 30 are transferred onto the sheet 100 in the Cyan-color toner transfer position. A voltage of 700V (direct current) is applied to a transfer roller 83. At this time, the electric charges move to a photosensitive body 30. Therefore, the potential difference between the front and back faces of the dielectric belt 50 is lowered from 200V to 0V.
Finally, the Black-color toners contained in the OPC drum 40 are transferred onto the sheet 100 in the Black-color toner transfer position. A voltage of 900V (direct current) is applied to a transfer roller 84.
In the above sequential transfer process, the potential difference between the surfaces of the sheet 100 and the photosensitive bodies 10 to 40 is always maintained at 1200V to obtain an even transfer efficiency.
However, viewing from the characteristics of the dielectric belt, it is necessary to keep the charge carrying function to lower the transfer voltage until at least the four-color transfer process is completed, so the resistance value of the dielectric belt must be high and constant. Therefore, the dielectric belt needs to be selected in a limited and permissible range, so that there is a problem that it is difficult to lower the apparatus cost.
It is known that when the running (printing) operation is executed to some degree, the surface-resistance on the dielectric belt as well as the electric charge carrying ability of the dielectric belt are lowered by the adsorption of impurities such as toners, etc. For example, FIG. 8 shows the result of measuring the electric potential fluctuations on the dielectric belt surface for the time (seconds) when a new dielectric belt (New Belt) before running and an old dielectric belt (Old Belt) after running during a specific time are charged to about 900V.
It is judged from this result that the electric charge carrying ability of the dielectric belt has been lowering. When the dielectric belt with the material characteristics in FIG. 8 is mounted onto the apparatus, the electric potential of the dielectric belt located in the toner transfer position is set to approximately 900V before running, but is lowered to approximately 500V after running, supposing that the electric potential of the dielectric belt is set to approximately 900V and it takes two seconds for the dielectric belt to be carried from the charging roller 70 to the transfer position. When the electric potential of the dielectric belt is lowered, the effective electric potential difference between the sheet 100 and the OPC drums is also lowered, so there is a problem that the transfer efficiency is lowered, depending on the apparatus running time (operation time).
Additionally, the tandem type color electro-photographic process contains many components, viewing from its characteristics that four image forming process units are parallel-disposed therein. For example, a general tandem type color electro-photographic process contains four sets of photosensitive bodies, photosensitive body chargers (containing the power source), exposure units, developing units, photosensitive body cleaning blades, transfer units (containing the power source), etc., respectively. Therefore, there is a problem that the tandem type color electro-photographic process is produced at higher costs than other color electrophotographic processes.
To reduce the number of components, it is considered as an example that a transfer power source should be used commonly. However, as explained above, when the electric potential of the dielectric belt is set to a high electric potential such as 1000V, etc., there are various electric potential differences between the front and back faces of the dielectric belt in the four-color transfer positions because of the material of the dielectric belt, the lowering of the surface-resistance value of the dielectric belt after running, the injection of electric charges from the photosensitive body into the dielectric belt. Therefore, the transfer efficiencies of the respective colors may be different from each other, when a voltage is applied from the same power source to the transfer unit.
FIG. 9 conceptually shows the result of measuring the transfer efficiency when the transfer voltage of each color is changed, in the case that the belt 50 is charged to approximately 1200V. As shown in FIG. 9, when the transfer voltages of all the colors are set to 1000V, the Magenda-, Cyan-, and Black-color toner transfer efficiencies are 100%, but the Yellow-color toner transfer efficiency is 80%. Therefore, there is a problem that the transfer power source could not be used commonly and the apparatus could not be produced at lower costs.
It is an object of the present invention to provide a color image forming apparatus and a color image forming method thereof, which enable a tandem type engine to be produced at lower costs.
It is another object of the present invention to provide a color image forming apparatus and a color image forming method thereof, which prevent the transfer efficiency from being lowered by running.
It is yet another object of the present invention to provide a color image forming apparatus and a color image forming method thereof, which enable a transfer power source to be commonly used and the apparatus to be produced at lower costs.
To attain the above objects of the present invention, a color image forming apparatus for forming a multi-color toner image to be a color image on a transfer material, or a color image forming method thereof comprises a plurality of image forming units for transferring toner images with different colors from an image carrier onto the transfer material, a dielectric belt for carrying the transfer material to sequentially pass the plurality of image forming units, a charging member for charging the transfer material to adsorb the transfer material onto the dielectric belt, and an electric potential setting member for setting the electric potential of the dielectric belt before the adsorption so as to prevent the electric potential difference between the front and back faces of the dielectric belt to affect each of the transfer operations.
According to the present invention, the electric potential difference between the front and back faces of the dielectric belt in the tandem type image forming unit can be set to approximately 0V before the dielectric belt adsorbs the transfer material so that the resistance value of the dielectric belt cannot affect the transfer operation. This enables the electric potential difference between the front and back faces of a dielectric belt to be set to approximately 0V in the respective toner color transfer positions. Therefore, there is a wide range of selecting the dielectric belt types, and the lowering of the transfer efficiency can be prevented, which is caused by the lowering of the surface-resistance of the dielectric belt after running.
According to the present invention, preferably by commonly using a transfer power source for transferring different color toners as well as by setting the above electric potential, the irregularities in the transfer efficiencies of the respective toner colors can be prevented, which are generated when the same transfer power source is commonly used, as well as the apparatus can be produced at lower costs.
Furthermore, according to the present invention, the transfer efficiencies of all toner colors can be set to appropriate values, preferably by setting the voltage applied from the above transfer power source so that the electric potential difference between the latent image and the transfer medium surface can be set between 1100V and 2600V.